Remote Sensing

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Remote Sensing Projections Datums
Instructor Michael Henderson Office
329-E E-mail mhender2_at_wvu.edu
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Why do we need to know about projections and
datums?
GIS databases use projections and datums to
overlay data properly. Georeference files are
used to overlay vector and raster datasets.
.proj Vector georeferencing file for ESRI .sdw
Georeferencing file for MrSID image. .tfw
Georeferencing file for GeoTIFF images.
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Map

• A map is a two-dimensional graphical
  representation of the surface of the Earth.
• Cartography is the science that deals with the
  construction, use, and principles behind maps.
• Two things must happen when a mapmaker constructs
  a map
• features in the real world must be
  "georeferenced" to a spheroid
• the spheroid must be projected onto the paper (or
  some virtual "surface" in a computer)

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Spheroid vs. Datum

• The spheroid models the shape of the earth's
  surface
• it is an idealization that does not account for
  local changes in topography
• The datum adds georeferencing to the spheroid
• it specifies where a clearly identifiable point
  on earth (the base point) should appear on the
  spheroid
• it shows where a base direction, such as north,
  points on the spheroid at the base point

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Map Projection
When projecting maps, the mathematical operation
shrinks and distorts parts of the spheroid to fit
on a flat piece of paper. This is where we get
the following saying all maps lie flat, and all
flat maps lie.
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Projection Distortions

• In the projection process, distortion is
  inevitably introduced
• 4 Earth/map properties are distorted
• area
• shape
• direction (angles)
• distance

• Distortion, then, is unavoidable
• However, different map projections can minimize
  distortion of one or another Earth/map properties
  

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Commonly Used Projections

• Small Scale Maps (1100,000 or larger)
• Conic projections work best for large areas
• Lambert Conformal Conic
• 2 standard parallels limits distortion to the
  edges
• ideal choice for the contiguous 48 states
• used by the State Plane Coordinate System
• Albers Equal Area Conic
• also 2 standard parallels
• also a good choice for the lower 48 states

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Commonly Used Projections

• Large Scale Maps (148000 or less)
• To map smaller areas, other projections are
  prominent, especially in government work
• UTM
• conformal projection used in some USGS maps
• also a coordinate system
• State Plane
• actually a coordinate system that uses either
  the UTM or Lambert Conformal Conic projection
• (WV has two zones for this projection. North
  South)

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Universal Transverse Mercator (UTM)

• Universal Transverse Mercator (UTM) coordinates
  define two dimensional, horizontal, positions.
• UTM zone numbers designate 6 degree longitudinal
  strips extending from 80 degrees South latitude
  to 84 degrees North latitude.
• UTM zone characters designate 8 degree zones
  extending north and south from the equator.

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The above coordinates describe the same area,
however, the first one is LL and the other is UTM.
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Projection Types

• Equal-area - correctly represents areas, but
  distorts angles and shapes on map margins
• Equidistant - distance/scale shown accurately
  along selected lines
• Azimuthal - accurately shows direction or angles
  from one central point to all others
• Conformal - generally preserves shape by showing
  relative local angles accurately

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Projection Categories

• Cylindrical--result from projecting a spherical
  surface onto a cylinder
• Conic--result from projecting a spherical
  surface onto a cone
• Azimuthal--result from projecting a spherical
  surface onto a plane
• Other--unprojected maps, compromise projections,
  and other examples

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DATUMS

• There are a few datums you will use more often
  than others
• for U.S. maps, these are NAD27 and NAD83
• for the most recent world maps, it is WGS84
• the World Geodetic System of 1984
• based on satellite measures of the earths
  shape/size
• an earth-centered datum
• the datum used for GPS measurements

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NAD 27

• The North American Datum of 1927
• Based on the Clarke spheroid of 1866
• an origin point at Meades Ranch, Kansas
• lots of control points calculated from
  observations in the 1800s--also, lots of errors
• Many USGS maps--and thus lots of spatial datause
  this datum
• The datum used with UTM in the U.S.

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NAD 83

• The North American Datum of 1983
• Based on the GRS80 spheroid
• used revised earth and satellite observations
• origin is the earths center of mass
• Much more accurate than NAD27
• some control points shifted as much as 500 ft
• Newer USGS data use this datum

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Introduction to GIS

• Geographic Information Science
• GIS is a combination of various disciplines to
  achieve a custom spatial data analysis system.
• All GIS systems differ from one company to the
  next. Each system should be design to meet the
  companys goals and objectives.
• A GIS system will not save you money, however, it
  will give your company a competitive edge.

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Four Input Layers
Input Type

• Point Vector
• Line Vector
• Polygon Vector
• Pixel Raster

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Ways to enter data into GIS

• Importing a spatial database or joining a
  non-spatial database to a spatial database.
• On-screen digitizing
• Exporting GPS data from Pathfinder Office to
  ArcView.
• Downloading data from GIS tech centers.
• Typically each state maintains a GIS data center,
  where people can access spatial data for free.

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GIS Tech Centers

• West Virginia www.wvgis.wvu.edu
• Pennsylvania www.pasda.psu.edu
• Ohio
• http//www.fws.gov/data/statdata/ohdata.html
• http//www.das.ohio.gov/ITSD/ESS/Gis/data.htm
• Maryland
• http//dnrweb.dnr.state.md.us/gis/data/data.asp